Rotating fan beam tomographic scanners normally comprise a circular area in which a patient or object to be examined is placed. A source of penetrating radiation such as X or gamma radiation is mounted to move in an arc adjacent the patient circle in order to rotate a beam of radiation at least partially around the patient. An arc of radiation detectors for measuring the intensity of radiation passing through the patient are positioned opposite the patient circle from the source of radiation. In some scanner models, an arc of radiation detectors rotates with the radiation source, while in others the detectors are stationary. A processing means transforms the intensity measurements of the arc of detectors into a visual display of the planar section of the patient being scanned.
Excessive radiation can cause two types of image degradation which is discussed in applicants' earlier filed application, now U.S. Pat. No. 4,277,685, the disclosure of which is incorporated herein by reference. As was taught in said earlier patent, scattered radiation travels in an unknown path through objects of unknown attenuation and creates erroneous data signals and noise. Hence, scatter can reduce the resolution of the image produced on the visual display. The '685 patent teaches means for shaping the cross-sectional dimensions of the radiation beam, specifically the incident beam, i.e., the beam emitted by the X-ray source before it is attenuated by the object being scanned. The '685 patent also noted that a detector (alternately referred to as receiver) collimator may additionally be used. The detector collimator serves the purpose primarily of eliminating scattered radiation from the attenuated transmitted beam and is the primary object of the present invention. In addition to eliminating scattered radiation caused by the object being scanned, the inlet opening in the detector collimator must correspond to the opening provided by the source collimator so that all the radiation to which the patient is subjected and which contains information usable in producing a representative image is utilized.
Image reconstruction algorithms normally assume a constant slice thickness. Thus, variations in the thickness of the beam will cause degradation and inaccuracies in the reconstructed image. It is therefore essential that the beam thickness be uniform throughout the fan. Previously, the slice thickness of the transmitted beam was determined by a pair of typically lead tipped aperture rings positioned immediately in front of the detectors and moved simultaneously together or apart relative to the center line of the X-ray beam in the Z axis. This conventional technique suffers from an accuracy problem since the large size of the rings causes them to flex and distort. Due to this distortion, the aperture opening can not be maintained accurate or uniform relative to the center line of the X-ray beam. The variations in the spacing between the two rings is particularly significant as the two rings are brought very close together. Thus, thin slices can not be made because of the dimensional deviations. In addition, the aperture opening varies with wear and tear of the rings.